diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 277848c2532..4061d1789cc 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-10-21  Gary Benson <gbenson@redhat.com>
+
+	* gdb.mi/mi-fullname-deleted.exp: Fix substituted
+	fullname test with Clang.  Also expand comments generally.
+
 2020-10-20  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.dwarf2/pr13961.S: Remove superfluous end-of-siblings marker.
diff --git a/gdb/testsuite/gdb.mi/mi-fullname-deleted.exp b/gdb/testsuite/gdb.mi/mi-fullname-deleted.exp
index f68bcc55bc3..e175041b46c 100644
--- a/gdb/testsuite/gdb.mi/mi-fullname-deleted.exp
+++ b/gdb/testsuite/gdb.mi/mi-fullname-deleted.exp
@@ -24,7 +24,17 @@ if [mi_gdb_start] {
 standard_testfile
 set srcfileabs [standard_output_file $srcfile]
 
-# "//$srcfile" It is used for the test of compare_filenames_for_search.
+# Double the final slash separator in $srcfileabs, such that
+# "/path/to/$srcfile" becomes "/path/to//$srcfile".  This is
+# passed as the "source" argument of gdb_compile, which, when
+# using GCC (as of 10.2) will result in the doubled slash
+# being incorporated into the built executable's debug info.
+# This is then used to check compare_filenames_for_search does
+# not falsely use an absolute filename as a relative one.
+# Note that Clang (as of 12.0.0) normalizes the path before
+# storing it in the executable's debug info, removing the double
+# slash.  This nullifies the "compare_filenames_for_search does
+# not match" test when using Clang (it can never fail).
 if { [regsub {/[^/]+$} $srcfileabs {/\0} srcfileabs] != 1 } {
     xfail "cannot double the last slash separator"
     return -1
@@ -36,6 +46,12 @@ if { [regsub {^(/[^/]+)/} $srcfileabs {\1subst/} srcfileabssubst] != 1
     return -1
 }
 
+# Generate a regular expression which to match $srcfileabs with
+# or without the doubled slash.  This is used by the substituted
+# fullname test.
+set srcfileabssubst_regexp [string_to_regexp $srcfileabssubst]
+regsub {//} $srcfileabssubst_regexp {\0?} srcfileabssubst_regexp
+
 set f [open $srcfileabs "w"]
 puts $f "int main (void) { return 0; }"
 close $f
@@ -54,10 +70,12 @@ mi_gdb_test "-interpreter-exec console \"set substitute-path ${initdir} ${initdi
 
 mi_gdb_test "-file-list-exec-source-file" ".*\",fullname=\".*\".*" "fullname present"
 
-mi_gdb_test "-file-list-exec-source-file" ".*\",fullname=\"[string_to_regexp $srcfileabssubst]\".*" "substituted fullname"
+mi_gdb_test "-file-list-exec-source-file" ".*\",fullname=\"$srcfileabssubst_regexp\".*" "substituted fullname"
 
-# Test compare_filenames_for_search does not falsely use absolute filename as
-# a relative one.
+# Test compare_filenames_for_search does not falsely use absolute
+# filename as a relative one.  Note that this test can falsely pass
+# with Clang, and possibly other compilers too; see the comment
+# above for more.
 mi_gdb_test "-break-insert -t /$srcfile:main" \
 	    "\\^error,msg=\"No source file named /[string_to_regexp $srcfile]\\.\"" \
 	    "compare_filenames_for_search does not match"